The infrared spectrum covers a range of wavelengths longer than the visible wavelengths but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The near infrared wavelengths extend from 0.75 micrometers to 10 micrometers. The far infrared wavelengths cover the range from approximately 10 micrometers to 1 millimeter. The function of infrared detectors is to respond to energy of a wavelength within some particular portion of the infrared region.
Heated objects radiate thermal energy having characteristic wavelengths within the infrared spectrum. Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. No. single detector is uniformly efficient over the entire infrared frequency spectrum. Thus, detectors are selected in accordance with their sensitivity in the range of interest to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detector must also be selected in view of the intended detection function.
A variety of different types of infrared detectors have been proposed in the art since the first crude infrared detector was constructed in the early 1800's. Virtually all contemporary infrared detectors are solid state devices constructed of materials that respond to infrared frequency energy in one of several ways. These include thermal detectors, photovoltaic detectors, and photo-conductive detectors.
Thermal detectors respond to infrared energy detectors by absorbing that energy causing an increase in temperature of the detecting material. The increased temperature in turn causes some other property of the material, such as resistivity, to change. By measuring this change the infrared radiation is measured.
Photo-type detectors (e.g.,. photoconductive and photovoltaic detectors) absorb the infrared frequency energy directly into the electronic structure of the material, inducing an electronic transition which, in turn, leads to either a change in the electrical conductivity (photoconductors) or to the generation of an output voltage across the terminals of the detector (photovoltaic detectors). The precise change that is effected is a function of various factors including the particular detector material selected, the doping density of that material and the detector area.
By the late 1800's, infrared detectors had been developed that could detect the heat from an animal at one quarter of a mile. The introduction of focusing lenses constructed of materials transparent-to infrared frequency energy, as well as advances in semiconductor materials and highly sensitive electronic circuitry have advanced the performance of contemporary infrared detectors close to the ideal photon limit.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements, the outputs of which are connected to sophisticated processing circuitry. By rapidly analyzing the pattern and sequence of detector element excitation, the processing circuitry can identify and monitor sources of infrared radiation. Though the theoretical performance of such systems is satisfactory for many applications, it is difficult to actually construct structures that mate a million or more detector elements and associated circuitry in a reliable and practical manner. Consequently, practical applications for contemporary infrared detection systems have necessitated that further advances be made in areas such as miniaturization of the detector array and accompanying circuitry, minimization of noise intermixed with the electrical signal generated by the detector elements, and improvements in the reliability and economical production of the detector array and accompanying circuitry.
Further difficulties are associated with conventional infrared detection systems designed for orbital use. Temperatures in space are extremely low. Though the detector elements may be designed to operate in a cryogenic environment, economic considerations may dictate that the associated processing circuitry operate in a higher temperature environment. The semiconductive materials and other elements incorporated into the processing circuit are typically more suited for room temperature operation. Where the detector circuitry is directly connected to the processing circuitry, it is difficult to thermally isolate the cryogenic detector circuit from-the room temperature processing circuitry. Connections communicating the signal interfaces, power supply connections, and other physical interconnections, typically effected by wires or metallized patterns, conduct heat and thereby transfers thermal conditions between the detector circuit and the processing circuitry. Moreover, such connections result in undesirable communication of noise and other transient signals between the detector circuit and the processing circuitry.
A problem commonly associated with circuits for interfacing detector elements with processing circuitry while isolating the detector elements and the related interface circuitry from the room temperature processing circuit is saturation of the photoreflective cell. Such saturation of the photoreflective cell results in lost signal information and thus provides incomplete and/or inaccurate data. As such, the dynamic range of the infrared detector elements is correspondingly limited by saturation of the photoreflective cell, thus substantially reducing the applications in which such infrared detector systems may be utilized.
One example of such a circuit for interfacing cryogenic detector arrays with processing circuitry so as to electrically and thermally isolate the infrared detector elements thereof from the signal processing circuitry is disclosed in U.S. Pat. No. 5,118,946, issued on Jun. 2, 1992 to Smith and entitled CIRCUIT FOR INTERFACING CRYOGENIC DETECTOR ARRAY WITH PROCESSING CIRCUIT, the contents of which are hereby incorporated by reference.
Thus, it would be beneficial to provide an optical coupler for infrared detection systems which is not subject to the detrimental effects of photoreflective cell saturation.